1. Field of the Invention
The present invention relates to an automatic exposure control system for an electronic flash camera, in general, and to a technique for employing the electronic flash in such a camera for determining certain photographic scene characteristics prior to exposure, in particular.
2. Description of the Prior Art
Automatic exposure control systems for electronic flash cameras that cause an electronic flash to illuminate a scene with artificial light prior to exposure, in order to determine certain photographic scene characteristics for the purpose of adjusting one or more camera functions, are known in the photographic arts. In, for example, U.S. Pat. No. 3,173,347 to Stimson et al., an automatic exposure control system for a camera is disclosed in which a source of artificial light is energized to illuminate a photographic scene prior to exposure and a signal derived from sensing a reflection of this illumination from a subject is employed to set a camera aperture.
In U.S. Pat. No. 4,302,084 to Greenwald et al, a focusing system for a photographic camera that also employs a pre-exposure flash is disclosed. This focusing system utilizes an electronic flash to direct light toward a subject to be photographed prior to exposure. The electronic flash light reflected from a subject within a scene is sensed and integrated. When the amount of reflected flash light reaches a predetermined level a quench circuit is activated to generate a signal which is used to extinguish the light emanating from the light source. A timer measures the elapsed time between the initiation of the electronic flash and the generation of the quench signal. Converter means are employed to translate the measured time interval into a range signal that is functionally related to the distance between the focusing system and the subject. This range signal may thus be employed to adjust or focus the movable element of an adjustable focus lens system.
A major advantage of the above-described ranging portion of the Greenwald et al. focusing system may be its ability to determine the distance to subjects over a relatively wide range of subject distances. However, this advantage produces a major disadvantage which is the inability to accurately determine subject distances of relatively close subjects. More importantly though, this type of ranging system consumes a relatively large amount of energy during the ranging process because light from the electronic flash must continue to illuminate a scene, including a remote subject located therein, until a predetermined level of reflected light has been sensed and integrated, a level that is dependent, in large part, upon subject distance. Generally, the more distant a subject the greater will be the energy consumption level.
The consumption of a large amount of energy for the pre-exposure flash is not a problem in a ranging system where two separate flash energizing or main discharge capacitors are provided, such as is provided in the electronic flash device described in U.S. Pat. No. 4,256,995 to Ishida. However, in an exposure control system employing an electronic flash unit that includes a single main discharge capacitor for storing all of the electrical charge needed to fire the electronic flash both before and during exposure, such as in the range-finding device described in the above-noted Greenwald et al. patent, an excessive consumption of this stored charge before exposure may excessively limit the amount of charge available to fire the electronic flash for the required amount of time during exposure.